Where are the Laplacian Lightning settings once the addon has been installed?
From the addon author's website:


  UI will be in >View3D>Tool Shelf>Laplacian Lightning (object mode)


Make it appear by pressing T (View>Tool shelf) while in 3d view and scroll down to the menu in the Tools tab. It is visible only in Object mode.

If this doesn't work, make sure you downloaded the proper version that match the current Blender version. See the website fore more details.